1. Field of the Invention
The present invention relates to to an image sensor, and more particularly, to a color image sensor.
2. State of the Prior Art
It is desirable to control the exposure in an image sensor such as a charge-coupled device (CCD). If an image sensor receives too much exposure, the resultant saturation and/or "blooming" destroys the spatial information and linearity of the video signal. If the exposure is too low, the signal-to-noise ratio and dynamic range suffer. Ideally, the maximum exposure is controlled to a point just below device saturation. The exposure can be controlled by either adjusting the source illumination (radiance), the collection optics (the solid angle subtended), or the time-duration of light collection (the integration time).
The integration time of CCD's can be controlled electronically. Typically, line time, i.e., the time between successive transfers of charge from the photodetectors to the shift registers, and the integration time are the same. However, there are sensors which electronically divert the photodetector charge to the substrate (so that no charge is collected) during the first portion of the line time, and then isolate the photodetector (so that charge is collected) for the the remainder of the line time. This is referred to as electronic exposure control or electronic shuttering.
Electronic exposure control has been previously used in monochrome linear CCD sensors. There is a problem, however, in using electronic exposure control for color image sensors. Integrated color linear CCD sensors have taken two forms. In one form, patterned color filters (such as a repeating red-green-blue pattern) have been placed on a single linear CCD. In another form, individual filters have been placed on three linear CCD's fabricated on a single substrate. A problem in controlling the exposure in both types of these sensors is that different colors should receive different integration times. One reason that different integration times are needed is that the red, green, and blue filters do not transmit the same photon flux in the pass bands for red, green, and blue, respectively. Another reason is that the light sources for scanners are not spectrally balanced; a tungsten light source, for example, is heavily weighted in the red.
One attempt at solving the problem of providing the proper exposure for each of the colors in a color image sensor is disclosed in U.S. Pat. No. 4,709,259, to Suzuki. This patent discloses a color image sensor which has a matrix of photodiodes each of which is adapted to sense red, green, or blue light. Charge signals stored in the photodiodes are transferred by means of MOS switches to three horizontal shift registers, one for each of the primary colors. Three vertical shift registers are provided to turn on the MOS switches for each color, and the charge storage time for each color is varied by varying the operating times of the three vertical shift registers. One of the main problems with the arrangement shown in the Suzuki patent is that the three colors cannot be clocked out in parallel, and thus, a complicated timing arrangement is necessary in order to process the signals.